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In a message dated 2/14/03 4:44:09 PM Eastern Standard Time, [EMAIL PROTECTED] writes: > That's only one choice. You can also level out and keep revving. My presumption is that we're going to stay legal and respect the redline operating limit. > The > Continental power curve for the C-90/O-200 mechanicals of the STC engine > show it putting out 85 hp at 2395 (or so) rpm. When it reaches 2575 rpm, I > see around 97 hp. If 85 hp will maintain that rpm and speed, then 97 hp > will allow that engine to rev past peak cam power output to whatever rpm > available power can maintain (but not increase further). I'm not sure where these numbers come from. Two things come to mind, though. First, at 2575 an O200 is rated about 80 hp , and secondly, the whole package has to be taken into consideration, including especially the carburetor and prop. The C85 carb is sized for the smaller displacement, and 2575 rpm. With the larger displacement, manifold pressure is constrianed by the size of the venturi (1 3/8"). More RPM is going to come only at further reduced manifold pressure. The O200 carb, on the other hand, is sized for the 200 cid and 2750 rpm (1.5" venturi). Put a C85 carb on an O200 and it'll make less power. (Again, the presumption is we're operating within the constraints of the TCDS and STC, and thus retain the C85 carb). > > The STC engine has a C-85 cam with peak power and torque characteristics > optimized for 2575 rpm. The C-90 has a cam with peak power and torque > characteristics optimized for its maximum continuous hp of 90 at only 2475 > rpm. So the STC engine should be capable of producing more power than even > the C90 curve shows from 2475 rpm up! I believe the prop and carb are the critical elements. The cam contributes, but to a lesser extent. After all, we're only talking 100 RPM difference here. > > I also don't think you're "legal" at 2575 rpm. According to the paperwork, you are. Nothing else matters :~) > The moment you allow this > powerplant/prop combination to exceed 2395 rpm, your C-85 produces more than > rated hp Not necessarily. RPM alone does not determine power. Manifold pressure has to be considered. Again, in a fixed pitch world, it's hard to quanitfy. As preveiously alluded to, the C85 produces 85 hp at 2575 rpm and about 28.92" MAP To produce 85 or more HP at lower RPM you'd need more MAP. That ain't gonna happen unless you have ram air or a turbo, or something like that. You do, however, have more compression and displacement with the modified engine, so you can produce some more power, or the same power at lower rpm. Exactly how much is not obvious, but the C90 information is more applicable than the O200. The C90 makes about 93 hp at 2575. There is no reason to think the C85 can make any more than that. Remember, we're already on the back side of the MAP curve. > - the FAA regards that as a no-no. Don't ask, don't tell. Actually, I recall the requirement is "within 10% of rated power". 85 + 8.5 is 93.5, so we're still good to go. > > The maximum static limit for the C-85 on an Ercoupe with a McCauley 1A90CF > or 1B90CM is 2225 rpm. If the STC does not require replacement or > repitching a C-85 prop to conform, the pilot decides to throttle-limit this > rpm to 2225 rpm until the wheels move (after that, you're not "static", > right?) or not. The STC engine starts with and sustains increased power. > Don't say this to loud. Here indeed is the rub. Technically, there should be another STC to allow installation of the modified engine in the airframe. This STC should raise the static RPM minimum and maximum slightly. By retaining the original limits, they have opened the window to repitch the prop to the point where the added power capability could actually be brought into play in normal flight regiemes. (There's that word again...) This is one reason why they chose to retain the original C85 operating limitations and power rating. To admit these had changed (outside the 10% limitation) would require an STC for each and every airframe/prop combination one installs the engine into. This process would, of course, be cost prohibitive and kill what is a really great STC. > C-85 power and rpm limitations are as unreasonable and arbitrary today as > the ones for the C-75 were in yesteryear. We already know mechanicals with > longer stroke (and higher stress) of substantially identical material and > production process are FAA APPROVED for sustained 2750 rpm (O-200). Static > limits merely give the FAA a measurable parameter of (in)efficiency...to > make sure that new Cessna 172 with 160 hp (you earlier used as an example) > can't go as fast as a 172 SP...different redline and prop AS APPROVED. Actually, static RPM limits are performance evaluators, not operating limitations. Checking the static RPM should be part of your regular condition evaluation process. An aircraft that cannot meet the lower limit either has too much prop, and won't be able to meet takeoff performance charts, or has a weak engine. An aircraft that exceeds the upper limit probably has too little prop pitch and will be difficult to keep below redline in cruise. Obviously the first condition is more problematic than the second. Oh, yeah, pilot limited static rpm is a meaningless. The aircraft must meet the static RPM limits in the TCDS at full throttle or, technically, it is unairworthy (and practically it will not perform as expected). Remember this the next time you are inspecting veins in tree leaves from above or run short of fuel 1/2 hour before you expect to... > Once again, the pilot with the STC engine has choices. He can exceed the > "paper" (and unrealistic) C-85 redline (with complete mechanical safety) as > far toward the O-200's maximum APPROVED sustained rpm of 2750 Not if he retains the prop and carb from the C85. There is no prop that meets the static limits that'll spin that fast anywhere other than a dive. Regardless, he's still limited legally by the redline. Anything else voids your insurance policy. > EITHER WAY the STC engine seems to offer substantial advantage. > See, we're in complete agreement! John ========================================================================== ==== To leave this forum go to: http://ercoupers.com/lists.htm Search the archives on http://escribe.com/aviation/coupers-tech/
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